The continuing development of new power transmission equipment gives rise to demands for new automatic transmission fluids capable of meeting increasingly severe performance requirements sought by the original equipment manufacturers and marketers of power transmission fluids. Among significant improvements in this regard are the ashless or low-ash synthetic base compositions described in U.S. Pat. Nos. 5,089,156 and 5,360,562 to D. R. Chrisope and R. J. Hartley. Those compositions, which utilize among other things mixtures of certain high and low viscosity hydrogenated poly-.alpha.-olefin oligomers and little or no high molecular weight viscosity index improvers, have excellent high and low temperature viscosity properties and excellent shear stability. Nevertheless, further progress in the field requires compositions which not only possess these properties, but which in addition exhibit superior seal performance and superior friction properties.